The present invention relates to making semiconductor materials and devices and more particularly relates to an apparatus for growing epitaxial layers for materials such as GaN.
Semiconductor wafers are frequently manufactured by placing wafers (also known as substrates) within a reaction chamber of a chemical vapor deposition (CVD) reactor and then growing one or more epitaxial layers on the wafers. During this process, wafers are placed inside the CVD reactor and reactant chemicals in gaseous form are introduced over the wafers in controlled quantities and at controlled rates for growing epitaxial layers on the wafers.
CVD reactors have various designs, including horizontal reactors in which wafers are mounted at an angle to the inflowing reactant gases; horizontal reactors with planetary rotation in which the reactant gases pass across the wafers; barrel reactors; and vertical reactors in which wafers are rotated at a relatively high speed within the reaction chamber as reactant gases are injected downwardly onto the wafers.
The reactant chemicals, commonly referred to as precursors, are typically introduced into the reaction chamber by placing the reactant chemicals in a device known as a bubbler and then passing a carrier gas through the bubbler. The carrier gas picks up molecules of the reactant chemicals to provide a reactant gas which is then fed into the reaction chamber of the CVD reactor using a mass flow controller.
The conditions under which the reactant gases are introduced into the reaction chamber have a dramatic effect upon the characteristics of the epitaxial layers grown on the wafers. These conditions, which may be modified to optimize the nature of the epitaxial layers grown on the wafers, typically include, density, vapor pressure, the flow path of the reactant gases, chemical activity and temperature. For example, the flow path of the reactant gases may be altered by changing the design of the flow flange used to introduce reactant gases into reaction chambers. In many instances, the properties of the epitaxial layers grown on the substrates are studied to determine the optimum flow path for growing a particular type of layer.
When depositing epitaxial layers on wafers, the wafers are typically placed on a wafer carrier within a reaction chamber, which, in turn, may be placed upon a rotatable susceptor. In certain designs developed by Emcore Corporation of Somerset, N.J., the susceptor and in turn the wafer carrier are heated by a source of heat located underneath the susceptor, such as resistive filaments or lamps. In these reactors, the growth of uniform epitaxial layers is attained by rapidly rotating the wafer carrier and susceptor on which the wafers are mounted. The thickness, composition and quality of the deposited layers determine the characteristics of the resulting semiconductor devices. Accordingly, the deposition process must be capable of depositing films of uniform composition and thickness on the front face of each wafer. The requirements for uniformity have become progressively more stringent with the use of larger wafers and with the use of apparatus which deposit coatings on several wafers simultaneously.
In deposition processes using conventional wafer carriers, the surface temperature of the wafers is usually cooler than the surface temperature of the wafer carrier as a result of the thermal resistance created by the susceptor, the interface between the wafers and the wafer carrier and the different emissivities of the materials from which the susceptor, wafer carrier and the wafer are made. Unfortunately, this temperature difference diminishes the quality of the resultant semiconductor wafers. For example, the higher temperature of the wafer carrier surface results in a nonuniform temperature on the surface of the wafers, particularly along their outer periphery, such that the layer(s) deposited along peripheral portions of wafers ordinarily are of inferior quality and limited value. Moreover, arrangements using a susceptor require too much filament power to heat the susceptor which then heats the wafer carrier.
In the typical prior art device shown in FIG. 1A, a wafer 10 is mounted atop a wafer carrier 12. In turn, the wafer carrier 12 is mounted on a susceptor 14 that is attached atop a rotatable support spindle 16. The wafer(s) 10, wafer carrier 12, and the upper end of susceptor 14 are generally located within an enclosed reactor chamber. A heating assembly 18 may be arranged below susceptor 14 for heating the susceptor, the wafer carrier 12 and wafer(s) 10 mounted thereon. Spindle 16 preferably rotates so as to enhance the uniformity of reactant gases flowing over the wafers 10. Rotating spindle 16 generally enhances the uniformity of reactant gases flowing over wafers 10 as well as temperature uniformity across the wafers 10.
Wafer carrier 12 includes circular-shaped pockets 20 on their upper surfaces 22 for holding wafers 10 in place as wafer carrier 12 is rotated during the deposition process. Typically, the circular pockets 20 have a diameter that is about 0.020″ larger than the diameter of the wafer(s) 10 and a depth that is about 0.002″ deeper than the thickness of the wafers. These wafer carriers 12 also typically include an annular flange 24 which is used for lifting and transporting wafer carrier 12 into and out of the reaction chamber. On its bottom surface, wafer carrier 12 may include an annular wall 26 for locating and holding wafer carrier 12 on susceptor 14 as the wafer carrier is rotated during the deposition process.
Referring to FIG. 1B, during the deposition process, wafers 10 are heated by heating assembly 18. As a result, earlier deposited epitaxial layers generally have a higher temperature than later deposited epitaxial layers. This frequently results in the peripheral edges 28 of each wafer 10 warping (i.e., curling up and away) from wafer carrier 12, as shown in FIG. 1B. As a result, the peripheral edges 28 of wafer(s) 10 are no longer in contact with wafer carrier 12 and are no longer being heated to the same level as interior portions of the wafer. Although the present invention is not limited by any particular theory of operation, it is believed that the warping is due to the bottom of the wafers being at a higher temperature than the top of the wafers or as a result of other stresses placed on the wafers during growth. Moreover, there is uneven heating of the wafers because the interior portions of the wafers are being heated when the curled outer portions of the wafers are further away from the heating assembly as a result of this wafer warpage. As a result, the epitaxial layers are not uniform across the wafers and the outer portions of the warped wafers 10 must be discarded. This is because, inter alia, semiconductor devices taken from the outer portions of the warped wafer will have different operating characteristics than semiconductor devices taken from interior regions of the wafers.
Another problem with the arrangement shown in FIGS. 1A and 1B is that the susceptor 14 prevents efficient heating of the wafer carrier 12. This situation is problematic when relatively high temperatures must be attained, such as when growing GaN wafers. At high temperatures, the heating filament may melt or deform.
Thus, there is a need for an apparatus that may be used to deposit more uniform epitaxial layers atop the entire surface of each wafer. More particularly, there exists the need for a wafer carrier that will maintain the wafers substantially flat during the epitaxial layer forming process, thereby preventing the edges of the wafers from curling.
There is also a need for an arrangement wherein the heating element directly heats the wafer carrier, without a susceptor or other object being located between the heating element and the wafer carrier. Such an arrangement enables the temperature of the heating filament to be maintained at a lower level, while still heating the wafer carrier to sufficient levels.